1. Field of Invention
This invention relates to wavelength-converted semiconductor light-emitting devices.
2. Description of Related Art
Semiconductor light-emitting devices including light emitting diodes (LEDs) are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. Sapphire is often used as the growth substrate due to its wide commercial availability and relative ease of use. The stack grown on the growth substrate typically includes one or more n-type layers doped with, for example, Si, formed over the substrate, a light emitting or active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. III-nitride light emitting devices efficiently emit UV through green light.
Illumination systems have been proposed which convert the color of light emitted by light emitting diodes by means of a fluorescent material such as a phosphor.
A dichromatic illumination system, which mixes the primary emission of a blue LED with light emitted by a yellow phosphor is described in U.S. Pat. No. 5,998,925. A Y3Al5O12:Ce3+ phosphor is coated on a III-nitride LED, and a portion of the blue light emitted from the LED is converted to yellow light by the phosphor. Another portion of the blue light from the LED is transmitted through the phosphor. Thus, this system emits both blue light emitted from the LED, and yellow light emitted from the phosphor. The mixture of blue and yellow emission bands are perceived as white light by an observer with a CRI between about 75 and about 80 and a color temperature, Tc, that ranges from about 6000 K to about 8000 K.
However, white light LEDs based on the dichromatic approach can only be used to a limited extent for general-purpose illumination, on account of poor color rendering caused by the absence of red color components.
A red-deficiency-compensating illumination system is illustrated in FIG. 1 and described in more detail in U.S. Pat. No. 6,351,069. LED 34 of FIG. 1 is designed to produce white output light that is well-balanced with respect to color to provide illumination for good color rendition. “The LED 34 includes the Gallium Nitride (GaN) die 12 that is positioned on the reflector cup lead frame 14 and is electrically coupled to the leads 16 and 18. The leads 16 and 18 provide excitation power to the GaN die 12. The GaN die 12 may generally be in a shape of a square. In the preferred embodiment, the GaN die 12 is configured to emit primary light having a peak wavelength of 470 nm, which lies within the blue region of the light spectrum, i.e., blue light. The GaN die 12 is covered by a spacing layer 36 made of a transparent material. The transparent material may be clear epoxy or glass.
“Adjacent to the spacing layer 36 is a fluorescent layer 38. The fluorescent layer 38 includes the fluorescent material 22 and a second fluorescent material 40. The fluorescent material 22 has a property to absorb the primary light and emit secondary light having a first peak wavelength, while the fluorescent material 40 has a property to absorb the primary light and emit secondary light having a second peak wavelength. Preferably, the secondary light emitted by the fluorescent material 22 has a broadband spectral distribution centered in the yellow region of the visible spectrum. However, the secondary light emitted by the fluorescent material 40 has a . . . spectral distribution that is intense in the red region of the visible spectrum. Thus, when the primary light and the secondary lights emitted by the fluorescent materials 22 and 40 are combined, white light is created that is rich in red color, in addition to other colors. The peak wavelengths of the secondary lights depend on the composition of the fluorescent materials 22 and 40, in addition to the peak wavelength of the primary light.
Layer 38 including the two fluorescent materials is a phosphor-resin mixture that “includes two fluorescent materials that are combined with a resin paste.” The phosphor-resin mixture “is deposited over the encapsulation layer to form a fluorescent layer that uniformly covers the encapsulation layer. The deposited phosphor-resin mixture may then be gelled, i.e., partially cured.” Thus, in the red-deficiency compensating system of U.S. Pat. No. 6,351,069, two fluorescent materials are mixed together, then suspended in a resin layer.